Self discharge has an _extremely_ strong function of temperature.

It is also a function of cell health, age, past abuse, etc. The list of factors that alter the rate of self-discharge is seeming endless.

Because it is such a strong function of temperature, small variations in the temperature of each the many cells in a high voltage pack can cause large imbalance because "self-discharge never sleeps". It chews away on the cells 24-7, regardless of whether they charging or discharging or simply open circuit. This is a problem because the end cells (that have a thick conductor to the outside world,) and cells on the outside edge of the pack, have a different thermal environment than do the inner-most cells. The temperature of the outside cells (and the end cells) is often starkly different than the inner cells. The self discharge is thus greatly different, and is typically dependent on the placement of the cell within the pack, and the difference between the outside temperature and the average pack temperature.

If your BMS happens to have some sort of cell voltage monitor or some sort of LED indicators on the cells themselves, this spacial imbalance becomes readily apparent. You can literally see the temperature variation that manifests itself as a SOC imbalance across the pack. You can watch the LEDs on perimeter of the pack light up before the LEDs on the inside of the pack when the outside temperature is cooler than the pack temperature. When the outside is warmer than the pack, the opposite is observed. It is like a topographic map of the cell temperature since the last charge. Even if the cell temperature is uniform at the time you actually charge and observe, the BMS LEDs will tell the tale of the cell temperature history since the previous full charge.

What is particularly insidious, is that contact resistance of the terminals, and internal resistance, also greatly effect the temperature of individual cells and thus elevate the self-discharge of those specific cells. This is why bad connections cause chronic cell imbalance and "weak" cells get out of balance. These cells run hotter than the rest, and the self-discharge skyrockets.

Bill D.

On 8/8/2017 6:23 PM, Hoegberg via EV wrote:

You might with some(all?) LFP even find a slight hysteresis in pack voltage, at 
exactly the same SOC..

  (most visible if you are in the 30-70% SOC-zone)
depending on ..if you have had a regen or a discharge pulse as your last event,
then the no load voltage seems not to be exactly the same, at the same SOC.
A higher rest voltage if you did a charge/regen pulse compared to if you just 
did a very short discharge.

I agree with the others, count Ah is the way ot go to know the SOC % in the 
flat part of the discharge curve,

Also my experience was, that decent cells dont have any / a lot of self 
discharge to balance out when in normal use, only milliamps might be needed 
over time, so if they are well (top)balanced once they seems to stay well 
balanced. But if the cells are damaged / have mfg problems from the begining 
then it might be a different situation,

Regarding balancers maximum current:
  we had a 5 Amp as the charger minimum current, so we did a pulse charge 
instead of use large balance currents,

So if one cell reach the "balancing" voltage then we can just stop the charger, 
and wait for that cell to reach its lower voltage, with only 100mA or so as balancer 
discharge current, then we re-enable the charger(5Amp) until any cell(s) again reach the 
balance-start voltage.

If you dont have any cell voltage monitoring , or any kind of signal / feedback 
from the balancers, then it might be tricky to do this, I dont have any good 
solution to shut of the charger in time if we dont know when we have a problem. 
(other than to use a lower charge current than your balancers can handle, but 
if one balancer do fail, then you will probably overcharge that cell later)

  I would prefer to use some kind of good cell voltage monitoring so you can 
get a warning in time if some cell go to low or to high, and also use it to 
shut of/cut down the charger, or cut back on the trottle if some of the cells 
get to low when driving.

in my opinion that should be a minimum when charging a large expensive pack..of 
more than 4 cells in series. :-)

If we only use the full pack voltage for the charger to decide whan to go in to constant 
voltage mode, then we can get in troubles, for example if one cell in the pack reach 
"full" and lift off almost like a capacitor, long before all the others have 
start to climb up faster in the end, so if all the other cells that still are the flat 
and lower voltage region the charger will give the pack and the already full cell its 
maximum current. Not good.
For example:
if we use 3.60 V as the chargers maximum cell voltage * 25 cells = 90 volt
what now if one cell is full and the others are still at 3.45 V each?

3.45 * 24 cells = 82.8 volt
Minus..say..89.8 Volt from the charger?
  = 1 cell will now try to reach up to about 7 Volt, and maybe still at full charger 
current...if so, that can probably be "bad".  :-)

/ John

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